Stability of second phase particles during processing by hydrostatic extrusion

Phase stability and transformations are well recognized for conventional materials of the grain in the micro-meters range. Nowadays, these phenomena attract new attention in the context of nanocrystalline materials in which the large fraction of atoms are located in interfaces area leading to the change of phase stability. In addition, such materials may be produced by the severe plastic deformation (SPD) which results in a large amount of defects such as dislocations being accumulated in nano-metals. It has been already shown that SPD strongly affects stability of carbides in Fe alloys. One should expect that it also affects the stability of particles in other multiphase materials, e.g. age-hardenable aluminium alloys in which the crucial factor influencing the mechanical strength is the presence of second phase particles.

The aim of the present work is to describe the stability of second phase particles during SPD processing and their effect on the process of grain refinement. Two age-hardened aluminium alloys (2017 and 7475) were subjected to HE process. The microstructure of the alloys before and after HE processing was observed using a transmission electron microscopy and described in terms of size and shape of grains and particles. During this processing the grain size was reduced below 100 nm. At the same time, the precipitates underwent the transformation from the plate-like shape into spheroids while inclusions were profoundly dispersed. The mechanisms of these changes have been proposed. Their influence on the process of grain refinement and high-angle grain boundary formation has been discussed.